The formation of the corpus luteum (which produces the majority of progesterone) is triggered by a surge in luteinising hormone production by the anterior pituitary gland . This normally occurs at approximately day 14 of the menstrual cycle and it stimulates the release of an egg from the ovary and the formation of the corpus luteum. The corpus luteum then releases progesterone, which prepares the body for pregnancy. If the egg is not fertilised and no embryo is conceived, the corpus luteum breaks down and the production of progesterone decreases. As the lining of the womb is no longer maintained by progesterone from the corpus luteum, it breaks away and menstrual bleeding occurs, marking the start of a new menstrual cycle.
Reference ranges for TSH may vary slightly, depending on the method of analysis, and do not necessarily equate to cut-offs for diagnosing thyroid dysfunction. In the UK, guidelines issued by the Association for Clinical Biochemistry suggest a reference range of - µIU/mL.  The National Academy of Clinical Biochemistry (NACB) stated that it expected the reference range for adults to be reduced to – µIU/mL, because research had shown that adults with an initially measured TSH level of over µIU/mL had "an increased odds ratio of developing hypothyroidism over the [following] 20 years, especially if thyroid antibodies were elevated".